From the viewpoint of classical fluid mechanics, turbulence is perceived as a chaotic condition with the excitation of all possible degrees of freedom. This perception is supported from direct observation of large scale activities, such as weather patterns in the atmosphere and water flowing in a pipe, to mundane experiences like stirring cream into coffee, or mixing paint.
Turbulence can be harmful or helpful: it produces undesired drag on a car or an airplane; but it effects mixing fuel with air in an engine, or rapidly distributing heat in a room. Turbulence has had an enormous impact on human experience, but its perception as a chaotic condition has resisted scientific analysis until recent years. With the advent of super computers which permit the numerical investigation of wall-bounded turbulent flow, and the performance of modern experiments, there has been a dramatic shift in the perception of turbulence. Where it was once viewed as being a chaotic condition, turbulence is now viewed as having coherent patterns of activity in the midst of apparent chaos.
Careful scrutiny of a wall or boundary of streaming turbulent flow, as for example, in the case of air flow over an airplane wing, or liquid flow inside a pipeline, has shown the presence of coherent structures in the form of pairs of counter-rotating streamwise rolls adjacent the walls, but located at the outer edge and beyond the sublayer. These rolls, which are sometimes referred to as streaks, show considerable meander and variation in their dynamic activity. Of great importance is their sudden contortion or kinking, resulting in a sudden bursting forth of slow moving fluid from near the wall into the fast moving main body of fluid motion. This bursting results in a net drag on the walls. It has been estimated that these bursts, which account for 80% of the drag on a wall, occur only roughly 20% of the time. Again, a study of the patterns of such flow shows that the contortion of the rolls undergoes a stereotypical coherence pattern through temporal change that is typical of all wall-bounded turbulence.
To specify the width of the streaks, it is first necessary to recognize that the streaks are a manifestation of local conditions beyond the sublayer of the flow adjacent to a wall, and not the nature of the wall, nor the flow field significantly spaced from the wall. Local conditions are fully specified by the average frictional stress at a wall, s, the density of the fluid, r, and the viscosity of the fluid, m. These quantities define a local size dimension, or length scale l.sub.* which is usually referred to as a wall unit and is equal to m/(sr).sup.1/2. The dominant roll diameter is roughly 100 wall units, or 200 l.sub.* per pair.
The term "dominant", in referring to the roll diameter, means that the greatest amount of turbulent energy (of the fluctuating velocity) resides in modes of activity of this size. There is, in addition, other modes of the same roll type, having a range of sizes and which also contain significant amounts of turbulent energy. In summary, the major contribution to drag on a wall arises because of the disruption of the orderliness of these roll type modes, to their contortion, and finally to the relatively violent bursting events that mixes slow moving fluid into more rapidly moving fluid.
This picture of the events in wall-bounded turbulence was significantly enhanced with the discovery that propagating structures are also present in the turbulent wall region. In reference (1) cited above, it is shown that propagating structures are coherent patterns which propagate at a constant group speed. In reference (2) cited above, the existence of propagating modes was further confirmed. As an aside, a literature search produced a paper written 20 years ago in which experiments on wall turbulence hinted at, but did not directly suggest, the presence and function served by such propagating modes in turbulent flow.
As it is argued in the above cited publications, the propagating modes act as triggers for the bursting events that give rise to the drag producing events found in turbulent wall-bounded flows. Although the propagating modes carry relatively little energy themselves, bursting events do not occur unless the propagating modes are present. In addition, the experimentally and numerically measured time courses of the bursting events corresponds to that of the propagating modes. The most energetic, and therefore the most important of the propagating modes, are those that propagate at an angle of about 65.degree. from the streamwise direction; and those in the range 50.degree.-80.degree. have the dominant energy content of the propagating modes.
The wavelengths of the triggering modes are also an important factor. Those waves with wavelengths comparable to the roll size play a significant role in the bursting events.
The most significant triggering modes have a lateral extent which is comparable to the wavelength of the energy bearing roll modes. This strongly implies the existence of a resonance mechanism which, through the triggering operation, facilitates ejection of the roll modes. For reference purposes, the main triggering modes are sometimes referred as the long wavelengths modes. There are no significant longer wavelength modes present, but many shorter wavelength modes are present.
It is therefore an object of the present invention to provide a method of and apparatus for modifying and managing turbulent flow through a modification of the trigger modes.